background colonies- i got the answer

Sergio Alonso sergioal at
Thu Oct 4 05:05:01 EST 2001

Baoshan ha escrito:

> Hi, i have got the answer--my ampicillin had be broken down. thanks a lot.

that's something i've seen before. I went to a foreing lab (just at the other
side of the Atlantic) for a short time visit, just three months. I was
carrying my beloved plasmids (months of clonning in my lab) to do some assays
in that lab. I electrotransformed some strains (maaany strains) with those
plasmids (maaany plasmids) and i got a very nice yield, plating in freshly
prepared ampicillin plates. The cells were grown in ampicillin suplemented
media (freshly prepared) and tested: ops!, incongruent results!!.. well, i did
some plasmid minipreps and, surprise!, no plasmid at all. "Ok, it could be the
miniprep solutions", i repeated them: no success. "Ok, maybe they lost the
plasmid, but they still grow in ampicillin...mmm. maybe the ampicillin stock i
prepared is failing"..."no problem, i'll prepare a new stock". So, i repeated
the whole thing three or four times before to start to think in strange
phenomenons, like chromosomal intagration (no, i performed som PCRs to check
that posibility), copy number decreasing by unknown reasons, contamination of
the electrocompetent cells... nothing worked.
Six weeks later, somebody in the lab saw how i was preparing a new ampicilllin
stock and ask me: "ah, are you using the can in the general antibiotics
freezer?". "Of course, why not?". "Because it's completely degraded. We use a
can which is the lab besides this one"..  !!??!!??!  why the &%&&/ they still
stored the degraded ampicillin and, even worse, in the general antibiotic tray
where the rest of the antibiotics were ok!!??
So, now i ALWAYS check the plates before doing any experiment.


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